Process plant monitoring, maintenance and control systems, like those used in chemical, petroleum or other processes, typically include one or more process controllers and input/output (I/O) devices communicatively coupled to at least one host or operator workstation, a maintenance workstation and to one or more field devices via analog, digital or combined analog/digital buses. The field devices, which may be, for example, valves, valve positioners, switches and transmitters (e.g., temperature, pressure and flow rate sensors), perform process control functions within the process such monitoring process variables or other physical phenomena for any reason, performing control functions such as opening or closing valves, measuring process parameters, etc. The process controllers, monitoring and maintenance applications receive signals indicative of measurements made by the field devices, process this information to implement a monitoring, a control or a maintenance routine, and generate control signals, maintenance instructions or other signals that are sent over the buses or other communication lines to operators or to the field devices to, for example, monitor a process, control the operation of the process or to perform maintenance tasks. In this manner, process controllers, monitoring applications and maintenance applications may execute and coordinate monitoring, control and maintenance strategies using the field devices via the buses and/or other communication links. Likewise, monitoring and maintenance applications may recognize problems and coordinate maintenance activities within the plant such as repairing devices, testing devices, detecting poorly performing devices, implementing calibration and other maintenance procedures, etc.
Process information from the field devices and the controllers may be made available to one or more applications (i.e., software routines, programs, etc.) executed by the operator or maintenance workstations (e.g., processor-based systems) to enable operator or maintenance personnel to perform desired functions with respect to the process, such as viewing the current state of the process (e.g., via a graphical user interface), evaluating the process, modifying the operation of the process (e.g., via a visual object diagram), tuning or calibrating devices, etc. Many process plant instrumentation systems also include one or more application stations (e.g., workstations) which are typically implemented using a personal computer, laptop, or the like and which are communicatively coupled to the controllers, operator workstations, and other systems within the process control system via a local area network (LAN). Each application station may include a graphical user interface that displays the process control information, monitoring information and maintenance information including values of process variables, values of quality parameters associated with the process, process fault detection information, and/or process and device status information.
In any event, sensors (also referred to as transmitters or process control devices) are generally disposed in a process plant at various locations to measure various process parameters or process phenomena, such as temperature, pressure, fluid flow, fluid level, etc. In other cases, samples may be taken from the process at various locations and these samples may be tested or analyzed offline to determine other types of process phenomena, such as PH level, viscosity, etc. Generally, the sensors or transmitters provide or communicate the measured or determined process parameter values to a control routine, a maintenance routine or device, a user interface or other device within the network for processing and/or display.
Due to their complexity, process plants typically require a large number of measurements to be made to ensure process safety and to provide adequate process control and monitoring information to plant systems and operating/maintenance personnel. In general, the more information that can be obtained at more measurement points, the better the plant can be made to operate. However, one hindrance to using more instrumentation is the total cost of installation. These costs include the purchase price, the installation cost and the system integration costs associated with the use of more measurement devices. If these costs can be brought down, users can afford more instrumentation.
Moreover, most sensors in use and installed in process plants today are intrusive in nature, in that these sensors must have some element thereof physically disposed in, connected to or touching the process fluid or other process element exhibiting the physical phenomenon which the sensor is to measure. As such, in many plant situations, it is only practical or possible to adequately install process parameter measurement devices, e.g., sensors, when constructing or installing the plant equipment. In other cases, installing a sensor at a particular point in the plant may require significant retrofitting of the plant equipment, at significant cost. In still other situations, it is impossible to place a sensor in a position at which the sensor can come into contact with and thereby measure the process fluid or process equipment at which the phenomenon is to be measured.
To alleviate these problems and to make installation of sensors easier in many situations, a number of non-intrusive sensors have been developed, which operate to measure a process parameter or process phenomenon without needing to come into direct contact with the process fluid or process equipment exhibiting the process phenomenon or at which the process phenomenon is to be measured. For example, temperature sensors have been developed to use infra-red light to detect the temperature of a vessel, wall or fluid, without needing the sensor to come into direct contact with that vessel, wall or fluid. Of course, other types of non-intrusive sensors exist. As a result, non-intrusive sensors are typically easier to install, and thus can reduce installation costs associated with the addition of more sensors in a process plant.
However, unfortunately, non-intrusive sensors tend not to be as accurate as traditional intrusive sensors with respect to measured process variables. Thus, while using measurement devices that do not require an intrusion into the process, but that instead make the measurement from outside a pressure boundary or outside the containment vessel, can substantially reduce installation and retrofitting costs, making measurements with such non-intrusive sensors will make the measurements less accurate. This fact will, in turn, make the measurements more suspect and less desirable in many uses, such as for process control purposes, maintenance decision making purposes, etc.